Hi,
That stem was too soft to go dormant. It probably was willing to grow more but got stopped by the colder temperature and less light due to days shortening.
I had one doing the same thing on my Sultane. Alas, nothing one can do against that.
It happened last year as my sultane was planted 12 months before from a pot. She had to adapt but that bud did not. It was growing strong and got stopped.
Humidity is your enemy in such situations.
Clean all leaves out of the shed. Try to pull them and the ones on the floor must come out or they will promote rotting and mold inside the shed .
In England, you shouldn't need the side walls; just the roof ... to stop the rain from getting in the pots.
As with my trashcan method, you need to keep the area vented and with fresh air .
But for that particular tree, I would blame it on the tree being pushed late with fertilizer. That often happens with young trees. As it happened to my Sultane.
This year the tree has adapted to its spot and to the growing season, and so far I don't have that problem... Perhaps I should go and check again ... Sultane is a soft strain for my Zone7.